It appears to me that you have the idea.
I have done this on other snaps but it appears that it will be difficult to make this work on the snaps on my Bianchi holsters as the place to hit with the punch is recessed into the thick leather, rendering it somewhat inaccessable. The idea is to place a dent in the edge of the snap so as to prevent the spring clip from moving in that directon. Thus, preventing the snap from opening from that side. This particular application may prevent access to the necessary portion of the snap.
In that case the existing snap could be drilled out and replaced. If you cannot find a directional snap, a regular snap could be modified before it is crimped in place.
You may want to find some old throw away snaps and practice a bit.
I hope this helps.